You can often buy keys directly from the developer. If you have an old physical CD copy but lost the manual, TaleWorlds support can sometimes help if you have proof of purchase.
Instead of risking your digital security on shady sites, there are several reliable ways to get a working serial key: mount and blade with fire and sword 1138 serial key verified
Searching for serial keys or "verified" activation codes for (specifically for version 1.138) often leads down a rabbit hole of outdated forum posts and risky websites. You can often buy keys directly from the developer